Oscillators are inherently non-linear circuits. FIG. 1 shows a basic oscillator 5. A passive lossy resonator 10 acts as the frequency selective element. Rp 20 represents the resonator losses. The Gm cell 30 is an active transconductance. The fixed bias voltage Vb sets the operating bias point of the active device and in turn the value of the transconductance Gm. The Gm cell 30 is connected in a positive feedback configuration and is used to provide a negative resistance (−1/Gm) that will cancel out Rp 20. When the product Gm·Rp is exactly equal to 1, the oscillation sustains. From a pure practical point of view, Gm must exceed 1/Rp for the oscillation to start. As the oscillation amplitude grows, Gm decreases due to the non-linear nature of the active transconductor. The oscillation settles when Gm·Rp=1 for the fundamental frequency component. At this point, the output current of the active Gm cell 30 injected into the resonator 10 has an appreciable amount of harmonics. These harmonics cause energy imbalance between the capacitor and the inductor and thus lower the oscillation frequency to restore balance as given in Equation 1:
                    f        =                              f            0                    ⁡                      (                          1              -                                                1                                      2                    ⁢                                          Q                      2                                                                      ⁢                                                      ∑                    2                    ∞                                    ⁢                                                                          ⁢                                                                                    n                        2                                                                                              n                          2                                                -                        1                                                              ⁢                                                                  (                                                                              I                            n                                                                                I                            1                                                                          )                                            2                                                                                            )                                              Equation        ⁢                                  ⁢        1            
In Equation 1, fo is the oscillation frequency without harmonics and Q is the intrinsic quality factor of the resonator. “I” denotes the current signal injected into the resonator where In is the amplitude of the nth harmonic of the current signal and I1 is the amplitude of the fundamental harmonic.
The level of current harmonics (In/I1) injected by the Gm cell 30 depends mainly upon the oscillation amplitude referred to the operating bias point of the active Gm cell. Since both, the amplitude and the operating bias point, exhibit a lot of variation versus electrical, physical and environmental conditions (together referred to as “operating conditions”), the harmonic level of the oscillator also varies, and thus the oscillation frequency f varies. This variation degrades the performance of clock reference oscillators with regard to the output frequency stability.
Traditionally, high performance clock reference oscillators have minimized the impact of harmonics on the oscillation frequency by decreasing the level of harmonics. This has been done by utilizing an automatic amplitude control (AAC) circuit (also called amplitude regulator circuit). The AAC is basically a feedback bias loop which sets the oscillation amplitude at an appropriate level and achieves the condition Gm·Rp=1 without driving the active Gm cell into a highly non-linear state.
FIG. 2 shows an oscillator circuit 100 containing an automatic amplitude control (AAC) circuit 110. Instead of having a fixed bias, the Gm cell bias Vb is controlled by the AAC 110. The AAC 110 comprises a peak detector 111 and a bias cell 112. The peak detector circuit 111 senses the AC (Alternating Current) oscillation signal Vo at the input and rectifies it into a DC (Direct Current) average signal VDC that is proportional to the oscillation amplitude Ao. According to the value of VDC, the bias cell circuit 112 generates a bias voltage Vb for the active Gm cell 30. The AAC modulates the transconductance Gm through Vb until the oscillation amplitude (Ao) settles at its target steady state value. The steady state solution of the circuit depends upon the characteristics of both the AAC 110 and the oscillator 5. Normally, the characteristics of both the oscillator 5 and the AAC 110 are adjusted such that the active Gm cell settles at a transconductance that is slightly higher than the critical oscillation condition Gm·Rp=1. For decades, this technique has been adopted in the design of high performance crystal oscillators (XOs) for two main reasons: (1) decreasing the current consumption of the oscillator and (2) decreasing the impact of harmonics on frequency stability. However, this technique does not offer good control over the absolute value of the oscillation amplitude.
In order to be able to precisely define the steady state oscillation amplitude, a modified AAC circuit has been utilized. FIG. 3 shows a schematic diagram of an oscillator circuit 200 whose amplitude is controlled by the modified AAC 210. The new AAC 210 comprises a peak detector block 111, an error amplifier 212 and a reference voltage VREF. The peak detector performs the same functionality that has been stated earlier. The error amplifier (EA) 212 compares VDC to VREF and generates the bias voltage Vb according to the error (VREF-VDC). The AAC 210 changes Vb and in turn Gm bias point until the oscillation amplitude (Ao) reaches a specific value that makes VDC equal to VREF. Hence, for a given value of VREF, the AAC 210 sets the appropriate operating bias point of the active Gm cell 30 so as to precisely achieve the oscillation amplitude (Ao) that corresponds to this specific value of VREF. FIG. 6, like FIG. 3, shows an AAC controlled oscillator 400. In FIG. 6, the oscillator is a CMOS LC oscillator.
The AAC circuit 210 has been used in oscillators for purposes other than frequency stability. For example, it is often used in inductor-capacitor (LC) voltage-controlled oscillators (VCO's) to maintain a constant output amplitude for the subsequent circuits and to optimally bias the VCO across the different operation conditions. In U.S. Pat. No. 7,659,788, a self-biased amplitude regulated LC VCO has been introduced; the aim was to optimize the VCO current consumption and to stabilize the oscillation amplitude versus the different operation conditions rather than stabilizing the output frequency which is not of interest in LC VCO's.